Magnetic Circular Dichroism in Archean Stratospheric Oxygen: Enantiomeric Excess of Amino Acids Produced in Volcanic Plumes.

In the terrestrial dipolar magnetic field, magnetic circular dichroism (MCD) of UV sunlight by paramagnetic O2 in an Archean atmosphere (mostly CO2 and N2) results in circular polarization anisotropy (~ 10-10). This is used to calculate enantiomeric excess (EE~10-13) of glyceraldehyde (3-carbon sugar) with a model that includes racemic production and asymmetric photolysis of its enantiomers. The sign and magnitude of enantiomeric excess (EE) vary with the Earth's latitude. Unlike random noise fluctuation in spontaneous mirror symmetry breaking (SMSB) models, the sign of EE is deterministic and constant over large areas of prebiotic Earth. The magnitude is several orders greater than the mean amplitude of stochastically fluctuating EE. MCD could provide the initial EE for growth of homochirality by asymmetric autocatalysis.

Magnetic optical activity (MOA) is induced in all matter by a longitudinal magnetic field. Magnetic optical rotation (MOR, the Faraday effect) and mag­ netic circular dichroism (MCD) arise respectively from the difference in refrac­ tive indices and absorption coefficients of left and right circularly polarized (CP) light due to the applied magnetic field. MOR occurs in both transparent and absorbing spectral regions ; MCD is an absorptive phenomenon only. MCD is the difference between left and right CP Zeeman spectra and provides no new information when the Zeeman components of a transition are resolved. However, MCD and MOR can be measured in broad bands where conventional Zeeman effects are undetectable, and their value lies essentially in extending the CP Zeeman experiment to broad spectra. At the beginning of the 1960s little was known about MOA in regions of broad absorption. Now, a decade later, the study of the dispersion of MOA through electronic absorption bands is a very active field of research in chemistry, physics, and biophysics. Instrumentation for the measurement of MOR and MCD and theoretical techniques for the analysis of MOR and MCD data have developed to a high level of sophistication. The nature of the phenomenon and the types of information to be obtained from its study are broadly understood. A general review of this field is therefore timely and is the purpose of this article. We discuss only MOA in absorbing regions and exclude MOR work in trans­ parent regions. The latter is (in principle) contributed to by all transitions of the system and is therefore generally less informative. Conventional CP Zeeman spectroscopy is also excluded. Emphasis is on MCD, since this has become the predominant technique; relevant MOR work is not excluded however. The scope of the article is determined by relevance to chemistry. Thus, color centers are discussed, since MOA work in this area has had a profound impact on work in chemistry; semiconductors, on the other hand, are excluded.

Magneto-optical Kerr effect and Faraday effect provide the basis of established methods for studying the magnetic properties of matter by polarized light in the visible spectral range. It was only quite recently that an analogous effect in the x-ray region, magnetic circular dichroism in x-ray absorption, was first observed by Gisela Schütz et al. for the near-edge fine structure at the K edge of ferromagnetic iron.1 Later on, magnetic circular x-ray dichroism (MCXD) was also observed at the LII,III thresholds of rare-earths2 and 3d transition metals,3 opening up the possibility for element-specific analyses of magnetic moments in compound magnets and multilayers. Today MCXD is mainly used as a tool at the LII,III x-ray absorption thresholds of 3d transition metals, where relatively large MCD asymmetries in the white lines upon reversal of either sample magnetization or circular polarization (photon spin) of the absorbed light are observed. MCXD can be understood in the simplest way in a one-electron picture by taking the spin polarization of the excited electron due to the inner-shell spin-orbit coupling (Fano effect4) into account as well as the spin-split density of final states at and above the Fermi level.5 More rigorous theoretical treatments have been given,6,7,8 which allow to recognize the three important ingredients for magnetic circular dichroism: (i) Exchange interaction as the driving force for long-range spin order; (ii) use of circularly polarized light with preferential propagation along the magnetic quantization axis; (iii) spin-orbit interaction providing the mechanism for an effective coupling between the angular momentum of the circularly polarized photon and the magnetically ordered electron spins.KeywordsCircular PolarizationMagnetic Circular DichroismExchange SplittingAntiparallel OrientationRare EarthThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Energy metabolism in anaerobic eukaryotes and Earth's late oxygenation

Tunneling dynamics of amino-acid: model for chiral evolution?

Light scattering by coexisting magnetic phases, that is domain structure, has been observed previously in various ferro‐ and ferrimagnetic crystals, CrBr3, EuO, MnBi and Y3Fe5O12 and in the antiferromagnets FeCl2 and Dy3Al5O12. Recently Griffin1 et al. reported that in FeCl2 at the metamagnetic transition the scattering differed for the two senses of circular polarization. They used this apparent ’’magnetic circular dichroism’’ to determine features of the magnetic phase diagram. Though an explanation was offered, the origin of this polarization asymmetry has remained a vexing puzzle. We have studied light scattering in (0001) sublimation sheets (20‐100μm thick) of FeCl2 with [0001] parallel to both the applied field and the direction of propagation of light. Asymmetry in the scattering of two circular polarizations was encountered throughout the spectral range 0.4−1.1 μm and the temperature range 2.1−20.0K. The diffraction and its dependence on circular polarization are explained in terms of the refracti...

<p>New particle formation (NPF) is an important source of aerosol particles at  global scale, including, in particular, cloud condensation nuclei (CCN). NPF has been observed worldwide in a broad variety of environments, but some specific conditions, such as those encountered in volcanic plumes, remain poorly documented in the literature. Yet, these conditions could promote the occurrence of the process, as recently evidenced in the volcanic eruption plume of the Piton de la Fournaise (Rose at al. 2019); a dominant fraction of the volcanic particles was moreover found to be of secondary origin in the plume, further highlighting the importance of the particle formation and growth processes associated to the volcanic plume eruption. A deeper comprehension of such natural processes is thus essential to assess their climate-related effects at present days but also to better define pre-industrial conditions and their variability in climate model simulations.</p><p>Sulfuric acid (SA) is commonly accepted as one of the main precursors for atmospheric NPF, and its role could be even more important in volcanic plume conditions, as recently evidenced by the airborne measurements conducted in the passive volcanic plumes of Etna and Stromboli (Sahyoun et al., 2019). Indeed, the flights performed in the frame of the STRAP campaign have allowed direct measurement of SA in such conditions for the first time, and have highlighted a strong connection between the cluster formation rate and SA concentration. Following these observations, the objective of the present work was to further quantify the formation of new particles in a volcanic plume and assess the effects of the process at a regional scale. For that purpose, the new parameterisation of nucleation derived by Sahyoun et al. (2019) was introduced in the model WRF-Chem, further optimized for the description of NPF. The flight ETNA13 described in detail in Sahyoun et al. (2019) was used as a case study to evaluate the effect of the new parameterisation on the cluster formation rate and particle number concentration in various size ranges, including CCN (i.e. climate-relevant) sizes.</p><p><strong>

Understanding of volcanic activity and its impacts on the atmosphere has evolved in discrete steps, associated with defining eruptions. The eruption of Krakatau, Indonesia, in August 1883 was the first whose global reach was recorded through observations of atmospheric phenomena around the world (Symons 1888). The rapid equatorial spread of Krakatau's ash cloud revealed new details of atmospheric circulation, while the vivid twilights and other optical phenomena were soon causally linked to the effects of particles and gases released from the volcano (e.g. Stothers 1996, Schroder 1999, Hamilton 2012). Later, eruptions of Agung, Bali (1963), El Chichón, Mexico (1982) and Pinatubo, Philippines (1991) led to a fuller understanding of how volcanic SO2 is transformed to a long-lived stratospheric sulfate aerosol, and its consequences (e.g. Meinel and Meinel 1967, Rampino and Self 1982, Hoffman and Rosen 1983, Bekki and Pyle 1994, McCormick et al 1995).

Some amino acids can crystallize from aqueous solution both as conglomerates and racemic compounds: under high supersaturation following rapid evaporation, dissolved amino acids draining over porous sand-bars behave like conglomerates whereas in the resulting deeper pool of water, amino acid solution switches to the more common racemic-compound system. We show how the two forms might have sequentially combined under prebiotic conditions to form the basis of homochirality. The paper is a quantitative analysis of enantiomeric excess (EE) this dual behavior of amino acids is capable of producing in tandem: Initial amplification by preferential crystallization (PC) in conglomerate system (CS) followed by further amplification in the racemic compound system (RCS). Using aspartic acid as a model system, ternary phase diagram shows that a minimum supersaturation of 1.65 is required in the CS for the solution-EE to reach its maximum value of 50% at the RCS eutectic point. A relationship is derived for the dependence of this threshold supersaturation on the eutectic solubilities of CS and RCS. For given supersaturation in CS, a relation is also derived for minimum solution-EE that must be produced by PC before CS switches to RCS. Required PC-induced threshold solution-EE of 0.194, 0.070, 0.033 is calculated for supersaturation of 2, 5, 10 respectively in aspartic acid. Switching from CS to RCS further amplifies solution-EE, resulting in an overall growth of aspartic acid solution EE from near-zero in CS to around 50% in RCS.

In this thesis, three new remote sensing instruments were designed and applied to characterize volcanic gas plumes: a passive scanning differential optical absorption spectrometer (DOAS), an active Long-Path DOAS, and an SO2-camera. Using the passive DOAS, sulfur dioxide (SO2) emission fluxes were successfully quantified at 6 volcanoes. At Kilauea (Hawaii), e.g., elevated fluxes were measured in March 2008, an indication of an upcoming eruption which occurred just weeks later. Halogen chemistry in volcanic plumes was also studied. Bromine monoxide (BrO) was positively detected at Stromboli (Italy), Popocatepetl (Mexico) and Masaya (Nicaragua) volcanoes. By applying an active DOAS system to volcanoes for the first time, BrO measurements were conducted at night at Masaya. A distinct diurnal cycle was found in which the highest concentrations were present around local noon while they were below the detection limit at night, a strong indication that BrO is photochemically formed in volcanic plumes. Finally, radiative transfer modeling was used to simulate the optical paths of photons during passive remote sensing measurements. The results show that an inaccurate assessment of radiative transfer can induce large errors in these measurements. A correction algorithm is presented that for the first time allows the retrieval of aerosol conditions in volcanic plumes from DOAS measurements.

Evolution of homochirality requires an initial enantiomeric excess (EE) between right and left-handed biomolecules. We show that magnetic circular dichroism (MCD) of sun’s ultraviolet C light by oxygen in Archaean earth’s anoxic atmosphere followed by chirally selective damage of biomolecules due to circular dichroism (CD) can generate EE of correct handedness. Our calculation of EE uses published data for CD of biomolecules and accepted magnitude for Archaean earth’s magnetic field. Independent of atmospheric oxygen concentration calculated EE has the same sign for all pyrimidine nucleosides which is opposite to that for amino-acids. Purine nucleosides have smaller EE values with opposite sign to pyrimidines but are less susceptible to UV damage. Homochirality is explained by origin of prebiotic life in one hemisphere of earth and its evolution to EE ~ ± 1 before reversal of terrestrial magnetic field. Chirality of biomolecules is decided by the direction of magnetic field where prebiotic life originated on Archaean earth.

Towards the detection of parity symmetry breaking in chiral molecules

Previous proposals for the origin of molecular homochirality, based on the effect of the weak neutral current (WNC) on enantiomers, and the amplification of the resultant parity-violating energy difference (PVED), are possibly flawed. The additive amplification of PVED in crystals and polymers ("Yamagata hypothesis") cannot lead to detectable levels of optical activity, the original theory apparently overestimating PVED by a factor equal to Avogadro's number. An alternative theory based on the irreversible and spontaneous evolution of a dynamically fluctuating system is apparently impractical. However, the nonlinear amplification of PVED via autocatalytic polymerization may be possible as indicated by a simplified physico-chemical approach. This may also occur during crystallization and melting, and form the basis of the second order asymmetric transformation. (Thus, reported differences in the melting points of enantiomers in several cases may well be real). Also, the preponderance of racemic compounds over conglomerates may be based on the destabilization of the conglomerate by the action of the WNC on the crystalline lattice. The WNC may also be involved in the anomalous scattering of X-rays, which possibly arises from their circular polarization: the current theory would need to be revised accordingly.

Giant magnetic circular dichroism (MCD) that shows a different response to incident wave with left or right-handed circular polarization under external magnetic field is promising for magneto-optical sensing, revealing symmetry and degeneracy information of electronic states. However, traditional methods and materials that are used to obtain significant MCD involve highly strong external magnetic field. Based on the excitation of subradiant plasmonic mode and Fano resonance in graphene oligomers in the mid-infrared region, we numerically demonstrate that MCD is enhanced three times larger than the previously reported method, based on the resonance of electric dipole plasmonic mode. This giant MCD is attributed to the remarkably different excitation efficiency of subdradiant plasmonic mode due to the interparticle coupling under left or right-handed circular polarization incidence and external magnetic field. Our results offer an effective mechanism to enhance MCD signal at the nanoscale, which facilitates the sensing, spintronic, nanophotonics and other such fields.

Saframycin A (SFM-A) is a potent antitumor antibiotic that belongs to the tetrahydroisoquinoline family. Biosynthetic studies have revealed that its unique pentacyclic core structure is derived from alanine, glycine, and non-proteinogenic amino acid 3-hydroxy-5-methyl-O-methyltyrosine (3-OH-5-Me-OMe-Tyr). SfmD, a hypothetical protein in the biosynthetic pathway of SFM-A, was hypothesized to be responsible for the generation of the 3-hydroxy group of 3-OH-5-Me-OMe-Tyr based on previously heterologous expression results. We now report the in vitro characterization of SfmD as a novel heme-containing peroxidase that catalyzes the hydroxylation of 3-methyltyrosine to 3-hydroxy-5-methyltyrosine using hydrogen peroxide as the oxidant. In addition, we elucidated the biosynthetic pathway of 3-OH-5-Me-OMe-Tyr by kinetic studies of SfmD in combination with biochemical assays of SfmM2, a methyltransferase within the same pathway. Furthermore, SacD, a counterpart of SfmD involved in safracin B biosynthesis, was also characterized as a heme-containing peroxidase, suggesting that SfmD-like heme-containing peroxidases may be commonly involved in the biosynthesis of SFM-A and its analogs. Finally, we found that the conserved motif HXXXC is crucial for heme binding using comparative UV-Vis and Magnetic Circular Dichroism (MCD) spectra studies of SfmD wild-type and mutants. Together, these findings expand the category of heme-containing peroxidases and set the stage for further mechanistic studies. In addition, this study has critical implications for delineating the biosynthetic pathway of other related tetrahydroisoquinoline family members.